This application relates to aerosol measurements, and more particularly, to measurements of cloud condensation nuclei.
The effect of human activities on climate is being recognized as one of the most important issues facing society (International Panel of Climate Change, 2001). Humans influence climate in numerous ways by cooling or heating the planet. Some components (such as greenhouse gas warming) are well understood and quantified; others are subject to high uncertainty. Aerosols (airborne particulate matter) belong to the latter category. It is believed that aerosols have a net cooling effect, but quantitative estimates are highly uncertain, of the order of the greenhouse warming effect itself. This uncertainty primarily originates from poorly understood aerosol-cloud interactions. Aerosols are the seeds for cloud formation, and those, around which droplets form, are called cloud condensation nuclei (CCN). The size, concentration, and affinity to water vapor of CCN can directly influence the size and concentration of cloud droplets.
Increasing the concentrations of aerosols (which occurs under polluted conditions) leads to more reflective and persistent clouds. Since clouds are very effective reflectors of incoming solar radiation, even small perturbations in their properties can significantly decrease the amount of solar radiation absorbed by the climate system, and thus lead to cooling, otherwise known as the aerosol “indirect effect”. Of all the components of climate change, the aerosol indirect effect is the most uncertain and potentially with the largest cooling effect. Until the aerosol indirect effect is well quantified, society is incapable of assessing its impact on future climate.
Measurements of CCN are fundamental for providing the link between cloud microphysics and the physical and chemical properties of aerosol. It is this liaison that is essential to improving our understanding of aerosol-cloud interactions and their subsequent effect on climate through modification of cloud radiative properties and the hydrological cycle. CCN measurements, however, are among the most challenging measurements in atmospheric sciences as obstacles in instrumental development and nuances in interpreting the data pose inherent problems. The primary source of problems lies in clouds themselves; they form in regions of very low water vapor supersaturation (by most a few tenths of a percent). Developing a technique that generates low supersaturation in a controlled manner, within in an ultralight package that responds quickly to ambient changes (necessary conditions for in-situ aircraft measurements), has proven to be challenging.
In addition, instrument development in this field has been largely empirical. As a result, measurements were often subject to unquantified uncertainty. Significant improvements in the measurement techniques are needed and this development constitutes an important step in this direction.
The ability of a particle to nucleate is at least in part determined by the saturation level of the environment, the size of the particle, and the chemical composition of the particle. When the relative humidity exceeds the saturation level where the vapor phase and the liquid phase are in equilibrium, a supersaturation state establishes and vapor begins to condense on surfaces and some particles. At a certain critical supersaturation, when the diameter of a condensation nucleus of a given chemical composition exceeds a critical diameter, the nucleus is said to be “activated.” Upon this activation, vapor can condense spontaneously on that nucleus and cause the nucleus to grow to a very large size which is limited only by the kinetics of condensational growth and the amount of vapor available for the condensational growth. The critical diameter at a given supersaturation usually changes with the chemical composition of the particles. Hence, particles of different chemical compositions can become activated at different sizes. One way to characterize condensation nuclei is to measure the critical supersaturation at which a particle activates. Various cloud condensation nucleus spectrometers have been developed for producing and measuring supersaturations in a desired range.
It is generally understood that cloud formation is determined by a subset of aerosol particles that grow into droplets by heterogeneous water nucleation. The ability of an aerosol particle to serve as CCN depends primarily on its size and soluble mass. The ratio of water vapor pressure at the surface of the droplet to that of a flat plane is the equilibrium saturation ratio SReq, and is described by the Köhler theory initially published by Köhler, “The nucleus in and the growth of hygroscopic droplets,” Trans. Faraday Sot., 32, 1152-1161 (1936). See also, e.g., Pruppacher and Klett, Microphysics of Clouds and Precipitation, Kluwer Academic Publishers, Boston, (1997) and Seinfeld and Pandis, Atmospheric chemistry and physics: From air pollution to climate change, 1326 pp., John Wiley, New York (1998). Two competing terms describe the Köhler equation; the surface tension term (i.e., the Kelvin effect) accounts for enhanced vapor pressure due to droplet curvature and scales to inverse diameter, Dp−1, and the dissolved solute term (i.e., the Raoult effect) depresses the water vapor pressure at the droplet surface and scales to Dp−3. The maximum SReq of the Köhler curve defines the critical supersaturation, Sc, and occurs at the droplet's critical diameter, Dpc. The droplet is in stable equilibrium with its environment when its diameter is less than Dpc. However, once the particle has activated (i.e., Dp>Dpc), the particle will continue to grow as long as the surrounding vapor pressure of water in the air is greater than the equilibrium vapor pressure of the solution droplet. The saturation ratios are often expressed as supersaturations, Sv, in percent (i.e., Sv (%)=(SR−1)×100%).
The shape of the Köhler curve dictates droplet growth and can be readily modified by surfactants and slightly soluble constituents in ambient aerosols. The presence of surface-active substances, such as water-soluble organic carbons (WSOC), can have a significant influence on the equilibrium vapor pressure by reducing the droplet's surface tension, which lowers Sc and enhances droplet growth. Slightly soluble compounds and soluble gases also affect the shape of the Köhler curve and may even allow the occurrence of stable, unactivated droplets of about 20 μm diameter in realistic, albeit polluted, conditions. Such modifications to the Köhler curve result in different growth rates of droplets and may impose challenges in defining activated and unactivated droplets and what constitutes CCN. Nonetheless, interpreting measurements from CCN instruments requires an understanding of these nuances and proper assessment of their importance in light of the particular experiment's focus.